A 100 ml round bottom flask fitted with a reflux condenser was charged with sodium 1,1,2-trimethyl-3-(3-sulfonatopropyl)-1H-benzo[e]indolium-6,8-disulfonate (3) (565 mg, 1 mmol), (E)-N—((Z)-2-bromo-3-(phenylamino)allylidene)benzenaminium bromide (5) (150 mg, 0.5 mmol), and pyridine (1 ml). Acetic anhydride (10 ml) was added to the flask, and the mixture was heated at 115° C. for 2 h, cooled to room temperature, and diluted with 25 ml of ethyl ether. The resulting dark blue dye precipitate was collected by filtration, then dissolved in 20 ml of water and purified by preparative reverse-phase HPLC to afford the compound 6 as a blue powder (285 mg, 50%; UV/vis absorption max 674 nm).
